No Safe Place

February
22, 2007: Imagine hiking across Antarctica, through
ice, cold and bitter wind, enduring months of hardship, and
finally arriving at the doorstep of the South Pole itself.

At
that moment you get hit by a Sahara sandstorm.

That's
the analogy scientists are using to describe what happened
to the ESA-NASA Ulysses spacecraft last December. "Ulysses
was approaching the South Pole of the sun when it was 'sandblasted'
by a cloud of high-energy particles—protons, electrons and
heavy ions," says Arik Posner, Ulysses Program Scientist
at NASA headquarters. The cloud was as foreign to the sun's
South Pole as a Sahara sandstorm would be to Antarctica.

The
strange tale begins on Dec. 5, 2006.

Astronomers
were in a state of excitement due to the sudden appearance
of a giant and angry-looking
sunspot on the sun's eastern limb—"sunspot 930,"
says Posner. On Dec. 5th it exploded, producing one of the
strongest solar flares of the past 25 years. On the "Richter
scale" of solar flares, X1 is considered intense; the
Dec. 5th flare was an X9. A flash of X-rays announced the
blast to sensors in Earth orbit, and moments later a cloud
of protons, electrons and heavy ions came rushing out of the
blast site. This is the cloud that pelted Ulysses.

The
process repeated on Dec. 6th (X6) and Dec. 13th (X3). Each
explosion created its own cloud of high-energy particles.
"We call these clouds 'radiation storms,'" says
Posner. "They are common after big flares."

What's
strange about these storms is where they went—to the South
Pole. "All three storms were detected by the Ulysses
spacecraft," says University of New Hampshire physicist
Bruce McKibben. He is principal investigator for COSPIN (Cosmic
and Solar Particle INvestigation), an array of sensors onboard
Ulysses that counts high energy particles. "The Dec.
6th event was particularly strong and rich in heavy ions."

The
Dec. 6th storm was so strong, in fact, "that if Earth
had been where Ulysses was, we would have experienced a full-fledged
Ground-Level Event," says Prof. Bernd Heber of the Institute
for Experimental and Applied Physics in Keil, Germany. In
other words, the particles were capable of tunneling all the
way through Earth’s atmosphere to reach the ground. Heber
is principle investigator for the Kiel Electron Telescope
(KET), a sensor onboard Ulysses able to detect such super-energetic
electrons, protons and ions.

Above:
Heavy ions (Z>2) counted by Ulysses over the sun's south
pole vs. ACE over the sun's equator in Dec. 2006. [More]

These
observations add up to "a big puzzle," says McKibben.
Sunspot 930 was near the sun's equator, while Ulysses was
over the sun's South Pole. The sun's magnetic field should
have kept the storms bottled up at low latitudes. How did
they reach Ulysses?

It's
a puzzle NASA is keen to solve. Solar radiation storms can
cause communication blackouts on Earth; they can disable satellites
in Earth-orbit; and in extreme cases they could be deadly
to astronauts. "We need to be able to predict the trajectory
of these storms," says Posner.

The
key is the sun's magnetic field. Just as Earth's magnetic
field guides compass needles, the sun’s magnetic field guides
radiation storms. "Radiation storms consist of charged
particles which naturally follow lines of magnetic force."

To
forecast the path of a radiation storm, researchers have in
the past relied on the "Parker spiral," a pioneering
magnetic model developed by University of Chicago physicist
Eugene Parker. According to his work, the sun's magnetic field
emerges radially from the sun's surface and spirals outward
into the solar system. "The spiral shape is caused by
the spinning motion of the sun," explains Posner. "It's
like a spiral stream of water from a spinning lawn sprinkler."

The
Parker spiral makes a straightforward prediction: Radiation
storms that begin near the equator should remain near the
equator. A storm might expand into the solar system and hit
Earth, which is not far off the sun’s equatorial plane, but
it should not hit Ulysses over the sun's South Pole.

Clearly,
there's more to the story than a graceful spiral. The real
solar magnetic field may contain kinks and twists that provide
a polar passage, a route storms can travel from equator to
poles. There is evidence for the idea: In 2000 and 2001, the
last Solar Max, the sun's magnetic field was full of convoluted,
non-Parkerian structures. "During that time, Ulysses
experienced six high-latitude radiation storms," notes
McKibben: data.

Mapping
and understanding these passages, if they exist, is work for
the future. Meanwhile, one thing is clear: "There is
no place in the inner solar system completely safe from radiation
storms," says Posner.

A
Cool Solar Mystery -- (Science@NASA) One pole of
the sun is cooler than the other. That's the surprising
conclusion of scientists who have been analyzing data
from the ESA-NASA Ulysses spacecraft.

South
Pole Flyby (Science@NASA) -- the ESA-NASA Ulysses
spacecraft is flying over uncharted territory, the mysterious
South Pole of the sun.

Cold
Peril -- (Science@NASA) The NASA/ESA Ulysses spacecraft
is perilously cold as it begins a newly extended mission
to study the sun.

A
Star with two North Poles -- (Science@NASA) Sometimes
the Sun's magnetic field goes haywire. The Ulysses spacecraft
has discovered how the effects are felt throughout solar
system.

Solar
Flares on Steroids -- (Science@NASA) Solar flares
that scorch Earth's atmosphere are commonplace. But
scientists have discovered a few each year that are
not like the others: they come from stars thousands
of light years away. The Ulysses spacecraft is crucial
for pinpointing these "solar flares on steroids."